Uv-stabilizers for siloxane systems

ABSTRACT

A UV-stabilizer comprising a mixture of hydroxybenzotriazole and a hydrolyzable silane containing at least one epoxy group is disclosed. The stabilizer is especially suitable as a component of a siloxane system for coating substrates, especially thermoplastic substrates and most especially polycarbonate substrates.

[0001] The present invention relates to non-volatile UV-stabilising mixtures for siloxane lacquer systems, which mixtures have certain hydroxybenzotriazoles as the UV-stabilising active structure and which are thus particularly suitable for the UV-stabilisation of thermoplastics, in particular of aromatic polycarbonates.

[0002] Materials are frequently protected from the harmful influences of the environment by providing them with a protective surface. Siloxane-based lacquers have proved particularly suitable for this purpose, inter alia providing the materials with a scratch-resistant surface.

[0003] These lacquers may contain so-called UV-stabilising substances in order to protect the lacquer itself and the underlying material, the so-called substrate, from harmful UV radiation. Apart from providing long-term UV protection, one requirement placed upon these substances is, inter alia, that they are not volatile so that they remain homogeneously distributed within the lacquer layer and do not escape from the lacquer layer either during curing or during subsequent use of the laquer. The UV-stabilising substances must furthermore not decompose rapidly, must be durably homogeneously miscible with the lacquers and the lacquer containing the UV-stabilising substances should be transparent.

[0004] U.S. Pat. No. 4,278,804 and U.S. Pat. No. 4,051,161 relate to UV-stabilising active substances and lacquers containing them. The substances disclosed therein, however, exhibit the disadvantage that they provide inadequate UV protection, they decompose too rapidly and/or the siloxane system containing the stabilisers has a yellow tinge.

[0005] U.S. Pat. No. 5,438,142 furthermore discloses the UV-stabilising active substance, 1-(3′-(benzotriazol-2″-yl)-4′-hydroxyphenyl)-1,1-bis(4-hydroxyphenyl)ethane. This active substance, however, exhibits the disadvantage that it is not durably miscible with siloxane-based lacquers.

[0006] The object thus arises of providing a UV-stabiliser system which does not exhibit the above-stated disadvantages.

[0007] This object is achieved according to the invention by the provision of UV-stabilising mixtures containing hydroxybenzotriazoles of the general formula (1) below and hydrolysable silanes containing epoxy groups.

[0008] R₁: H, C₁-C₁₈ alkyl, C₅-C₆ cycloalkyl, C₆-C₁₂ aryl,

[0009] R₂: H, halogen, preferably Cl, or C₁-C₁₂ alkyl,

[0010] R₃: a single bond, C₁-C₁₂ alkylene, C₅-C₆ cycloalkylene or phenylene,

[0011] R₄: H, alkali metal, ammonium, alkaline earth metal, C-C₁₂ alkyl,

[0012] The present invention furthermore provides UV-stabilising mixtures having a molar ratio of epoxy groups of the silane to the hydroxybenzotriazole of the general formula (1) which is greater than 1.4, preferably greater than 2, particularly preferably greater than 8. The molar ratio of epoxy units of the silane to the hydroxybenzotriazole of the general formula (1) should not, however, exceed 1:100.

[0013] The mixtures according to the invention are suitable for the UV-stabilisation of siloxane systems, in particular of scratch- and abrasion-resistant siloxane coating

[0014] The mixtures according to the invention are suitable for the UV-stabilisation of siloxane systems, in particular of scratch- and abrasion-resistant siloxane coating materials. Such UV-stabilised coating materials, preferably lacquers, may be used for coating materials of all kinds, such as for example wood, textiles, paper, stone articles, but preferably for coating plastics, metals, glass and ceramics, particularly preferably for coating thermoplastics and very particularly preferably for coating polycarbonates.

[0015] The hydroxybenzotriazoles used for the non-volatile, UV-stabilising mixtures according to the invention are compounds of the general formula (1).

[0016] Preferred compounds of the formula (1) are:

[0017] The compounds of the formula (1) are either known from the literature or obtainable using processes known from the literature, for example in accordance with the reaction scheme disclosed on page 7 of EP-0 057 160. This patent application is introduced as a reference and is accordingly part of the disclosure of the present invention.

[0018] Silanes containing epoxy groups are generally taken to mean compounds which, on the one hand, possess at least one epoxy ring and simultaneously have groups which form silanol structures under hydrolysing conditions.

[0019] Epoxysilanes as are preferably used according to the invention are described, for example, in U.S. Pat. No. 2,946,701. They are compounds of the formulae (2) or (3):

[0020] R₅ is a divalent hydrocarbon residue having at most 9 carbon atoms or a divalent residue having at most 9 carbon atoms consisting of C, H and O atoms, wherein the O atom is present as an ether bond residue.

[0021] R₅ is preferably —CH₂OCH₂CH₂CH₂—.

[0022] R₆ is an aliphatic hydrocarbon residue having at most 4 carbon atoms, an acyl residue having at most 4 carbon atoms or a residue of the formula (CH₂CH₂O)_(n)Z, in which n is at least 1 and Z means an aliphatic hydrocarbon residue having at most 4 carbon atoms;

[0023] m is 0 or 1.

[0024] Production of these epoxysilanes is also described in U.S. Pat. No. 2,946,701. This patent is accordingly introduced as a reference. Particularly preferred epoxysilanes are those compounds in which R₆ is methyl. They are commercially available, inter alia from the companies Union Carbide and Hüls AG as:

[0025] A-187 or Dynasilan Glymo 3-glycidyloxypropyltrimethoxysilane

[0026] A-186 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

[0027] Production of the UV-stabilising mixtures

[0028] The UV-stabilising components are produced by homogeneously mixing compounds of the general formula (1) with the hydrolysable silanes containing epoxy groups and heating this mixture. Heating should be performed for at least 30 minutes at at least 90° C. The temperature should preferably be above 120° C. during heating. It has proved particularly favourable to use a mixing ratio at which stoichiometrically more epoxy groups are present than the —R₃—CO—OR₄— groups of the hydroxybenzotriazole of the general formula 1. The molar ratio of epoxy units of the silane to the hydroxy-benzotriazole of the general formula (1) should thus be greater than 1.4, preferably greater than 2, particularly preferably greater than 8.

[0029] The UV-stabilising components need not necessarily be produced separately so that they may subsequently be added to the siloxane system to be stabilised, but may also be synthesised in situ as a sub-stage during synthesis of the siloxane systems/siloxane coating materials.

[0030] Siloxane systems/siloxane coating materials

[0031] The siloxane systems are substantially thermally curing systems which preferably crosslink by a condensation reaction to yield —Si—O—Si— linkages. Other crosslinking mechanisms may proceed in parallel. Such systems are described, for example, in U.S. Pat. Nos. 3,790,527, 3,865,755, 3,887,514, 4,243,720, 4,278,804, 4,680,232, 4,006,271, 4,476,281, in DE-A 4,011,045, 4,122,743, 4,020,316, 3,917,535, 3,706,714, 3,407,087, 3,836,815, 2,914,427, 3,135,241, 3,134,777, 3,100,532, 3,151,350, in DE-A 3,005,541, 3,014,411, 2,834,606, 2,947,879, 3,016,021, 2,914,427 and 4,338,361 and should be considered part of the present disclosure.

[0032] The present invention accordingly also provides siloxane systems UV-stabilised according to the invention.

[0033] Preferably used siloxane systems are those containing particulate material selected from among oxides, oxide hydrates, nitrides and carbides of Si, Al, Sb and B and of transition metals, preferably Ti, Ce, Fe and Zr, and having a particle size in the range from 1 to 100 nm, preferably from 2 to 50 nm.

[0034] The UV-stabilising mixture according to the invention should be added to the siloxane system in such a quantity, relative to the solids content of the siloxane system, that the proportion of hydroxybenzotriazole, relative to the solids content of the siloxane system, is 0.3 to 20, preferably 3 to 15, particularly preferably 5 to 10 wt. %.

[0035] Reference is made to DE-A 2 914 427 and DE-A 4 338 361 with regard to the production of siloxane-based scratch-resistant coating systems and components thereof and these documents are thus part of the present description.

[0036] Substrates, materials

[0037] The siloxane systems provided with the UV-stabilising mixture according to the invention may be used as bulk materials and as coating materials. There are no restrictions as to the substrate materials which may be selected for coating. These UV-stabilised coating materials are preferably suitable for coating wood, textiles, paper, stone articles, metals, glass, ceramics and plastics and in particular for coating thermoplastics, as are for example described in Becker/Braun Kunststoffhandbuch, Carl Hanser Verlag, Munich, Vienna, 1972. They are very particularly suitable for coating transparent thermoplastics, preferably polycarbonates.

[0038] Conventional coating processes are used for coating purposes, for example dipping, flooding, pouring, spinning, spraying or brushing.

[0039] The coating is applied to film thicknesses of, for example, 2 to 200 μm, preferably of 2 to 30 μm and particularly preferably of 5 to 15 μm. The substrate may optionally be primed with a coupling agent or primer coat before application of the coating.

[0040] The lacquers are preferably cured at temperatures of >90° C.

[0041] For the purposes of the present invention, thermoplastic, aromatic polycarbonates include both homopolycarbonates and copolycarbonates; the polycarbonates may, in a known manner, be linear or branched.

[0042] A proportion, up to 80 mol %, preferably of 20 mol % to 50 mol % of the carbonate groups in the suitable polycarbonates may be replaced by aromatic dicarboxylic acid ester groups. Such polycarbonates, which contain both acid residues of carbonic acid and acid residues of aromatic dicarboxylic acids incorporated in the molecular chain, are more accurately termed aromatic polyester carbonates. They are to be subsumed within the superordinate term of thermoplastic, aromatic polycarbonates.

[0043] Details of the production of polycarbonates have been described in hundreds of patents over the past approx. 40 years. Reference is made, merely by way of example, to “Schnell, Chemistry & Physics of Polycarbonates”, Polymer Reviews, volume 9, Interscience Publishers, New York, London, Sydney 1964, to D.C. Prevorsek, B. T. Debona & Y. Kesten, Corporate Research Center, Allied Chemical Corporation, Morristown, N.J. 07960, “Synthesis of poly(ester carbonate) copolymers” in Journal of Polymer Science, Polymer Chemistry edition, volume 19, 75-90 (1980), to D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne', Bayer A G, “Polycarbonates” in Encyclopedia of Polymer Science & Engineering, volume 11, second edition, 1988, pages 648-718 and finally to Dr. U. Grigo, Dr. K. Kircher & Dr. P. R. Müller “Polycarbonate” in Becker/Braun, Kunststoff-Handbuch, volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester, Carl Hanser Verlag, Munich, Vienna, 1992, pages 117-299.

[0044] The thermoplastic polycarbonates have average molecular weights {overscore (M)}_(w) (determined by measuring relative viscosity at 25° C. in CH₂Cl₂ at a concentration of 0.5 g per 100 ml of CH₂Cl₂) of 12000 to 400000, preferably of 18000 to 80000 and in particular of 22000 to 60000.

[0045] The present invention accordingly also provides coated materials, preferably polycarbonate and particularly preferably polycarbonate provided with a scratch-resistant coating.

EXAMPLES Example I

[0046] a) UV-absorbing starting materials

[0047] a1) 2-(2-hydroxy-3 -tert.-butyl-5-(2-carboxyethyl)phenyl)benzotriazole, m.p. 195° C. (produced as in EP 0 057 160, Example 1)

[0048] a2) 2-(2-hydroxy-3-cyclohexyl-5(3-carboxypropyl)phenyl)benzotriazole

[0049] α) 132 g (0.75 mol) of 2-cyclohexylphenol are dissolved in 800 ml of dry chlorobenzene. 200 g (1.5 mol) of AlCl₃ are then added at 5 to 10° C. followed at 0 to 10° C. by a solution of 73.5 g (0.75 mol) of maleic anhydride in 400 ml of chlorobenzene. After 12 hours at room temperature, the mixture is poured into iced water and acidified with concentrated HCl. 85 g of a powder having an m.p. of 187 to 190° C. are obtained.

[0050] β) 34.5 g (0.25 mol) of o-nitroaniline are stirred in 300 ml of water and 69 ml of concentrated HCl. A solution of 17.3 g (0.25 mol) of sodium nitrite in 155 ml of water is then added dropwise at 5° C. This solution is then added dropwise at 5° C. to a solution of 68.5 g (0.25 mol) of compound α and 79.5 g (0.75 mol) of sodium carbonate in 1 liter of water. 117 g of a solid having an m.p. of 155° C. are obtained.

[0051] γ) 42.5 g (0.1 mol) of the azo dye β are combined with 200 ml of 2 n NaOH. 50 g of zinc powder are then added and 80 ml of 10 n NaOH are run in within 1 hour such that the temperature remains below 45° C. The mixture is then heated to 90° C. for 4 hours, filtered and the filtrate acidified with HCl.

[0052] After recrystallisation from cyclohexane, 31 g of 2-(2-hydroxy-3-cyclohexyl-5-(3-carboxypropyl)phenyl)benzotriazole are obtained as colourless crystals of an m.p. of 165° C.

[0053] b) UV-stabilising component prepared from a1 and 3-glycidyloxypropyltrimethoxysilane (Glymo)

[0054] 50 g of a1 and 450 g of 3-glycidyloxypropyltrimethoxysilane are introduced into a vessel and heated to 140 to 150° C. under a nitrogen atmosphere while being stirred and are maintained at this temperature for one hour. This mixture (mixture 1) was varied as follows: Mixture: 2 100 g a1 400 g Glymo 3 150 g a1 350 g Glymo 4 200 g a1 300 g Glymo 5 100 g a1 400 g α-(3,4-epoxycyclohexyl)ethyltri- methoxysilane

[0055] c) Production of the siloxane coating material according to DE-A 2 914 427 (coating sol I)

[0056] α) 19.8 g of glacial acetic acid, 210 g of distilled water and 227 g of isopropanol are added to 300 g of colloidal silicic acid having an SiO₂ content of 30 wt. %. After thorough mixing, 900 g of methyltriethoxy-silane are added and the mixture heated to 60° C. while being stirred. The mixture is left at this temperature for 4 hours and then a further 1200 g of isopropanol are added to the mixture. Once the product has cooled to room temperature, the slightly opaque solution is filtered.

[0057] β) 340 g of isopropanol, 190 g of tetraethoxysilane and 360 g of methyltriethoxysilane are introduced into a vessel fitted with a stirrer and reflux condenser. This mixture is combined with 180 g of 0.05 n hydrochloric acid and co-hydrolysed by refluxing for five hours. The mixture is cooled to room temperature after the reaction. A solution is obtained which is a partial hydrolysate of tetraethoxysilane (5.1%, calculated as SiO₂) and a partial hydrolysate of methyltriethoxysilane (12.6%, calculated as CH₃SiO_(1.5)).

[0058] Before use as a coating material, the two components α) and β) are mixed together in a 1:1 ratio and dissolved in a mixture prepared from 60 parts by weight of n-butanol, 40 parts by weight of acetic acid and 20 parts by weight of toluene.

[0059] d) Production of a siloxane coating material according to DE-A 4 338 361 (coating sol II)

[0060] A boehmite sol was produced by combining 12.82 g of acetic acid-stabilised (6.4 wt. % acetic acid) boehmite powder with 104.62 g of 0.1 n HCl. Subsequent ultrasonication (20 minutes) produced a transparent, colourless solution, 24.3 g of which were combined with a mixture prepared from 118.17 g of GPTS (3-glycidyloxypropyltrimethoxysilane) and 62.50 g of TEOS (tetraethyl orthosilicate). The reaction mixture was stirred for 2 hours at room temperature and then, while being cooled with ice, combined with 18.93 g of aluminium tributoxyethanolate. The resultant clear sol was stirred for 2 hours at room temperature and then, while being cooled with ice, combined with 93.14 g of the above boehmite sol and 79.30 g of butoxyethanol.

[0061] e) UV-stabilised coating sols I and II

[0062] A 60 g portion of the UV-stabilising mixture 2 according to the invention was added to a 1000 g portion of each of coating sols I and II. Silica glass was coated with these compositions and UV light transmission measured with a Beckmann DU 70 photometer in the wavelength range from 250 to 600 mn. The coating film was 5 μm thick and absorbed >98% of the radiation of a wavelength of <350 nm critical for polycarbonate.

[0063] Coating of substrates and testing of coating properties

[0064] Bisphenol A polycarbonate sheets (T_(g)=147° C., M_(w) 27500) of dimensions 105×150×4 mm were cleaned with isopropanol and primed by dipping in a mixture prepared from 3 wt. % of aminopropyltrimethoxysilane and 97 wt. % of ethylene glycol monobutyl ether followed by 30 minutes' heat treatment at 130° C. The sheets were then provided with a 20 μm film of one of coating sols I or II at a dipping speed V=100 cm/min. After flashing off for 10 minutes at room temperature, the coated sheets were dried for 1 hour at 130° C. The film thickness of the scratch-resistant lacquers was approx. 5 μm after drying. Once curing was complete, the coated sheets were stored for 2 days at room temperature and then exposed to a defined quantity of UV radiation.

[0065] UV exposure testing

[0066] The polycarbonate sheets were exposed to filtered xenon arc radiation with a water spray cycle to DIN 53387-1-A-X under the following test conditions: Weathering apparatus: Xenon-WOM Radiation intensity at 340 nm: 0.35 W/m² (preferably) Filter combination: inner: Pyrex, outer: Pyrex Blackboard temperature: 60° C. ± 5° C. Black standard temperature: 65° C. ± 3° C. Mode of operation: constant Water spray cycle: 102:18 Relative atmospheric humidity: 60-80%

[0067] Yellowing as a function of exposure time was used as the evaluation criterion for the weathering resistance of the lacquer-coated sheets. The corresponding yellowness of the sheets was determined as the Yellowness Index (Y.I.) to ASTM D 1925-70.

[0068] Y.I. values after Xenon-WOM 102:18 weathering 1000 2000 3000 5000 Specimens 0 h h h h h Polycarbonate with UV-stabilised 2.1 2.2 2.7 2.8 4.3 coating sol I according to mixture 2 Polycarbonate with UV-stabilised 2.5 2.7 3.2 3.3 4.8 coating sol II according to mixture 2 Comparison Polycarbonate with coating sol I 1.8 2.2 6.4^(a)) 7.6^(a)) —^(b)) without UV stabilisation Polycarbonate with coating sol II 1.9 2.6 6.3^(a)) 7.9^(a)) —^(b)) without UV stabilisation 

1. UV-stabiliser mixture containing A) hydroxybenzotriazole according to the general formula (1):

R₁: H, C₁-C₁₈ alkyl, C₅-C₆ cycloalkyl, C₆-C₁₂ aryl, R₂: H, halogen, preferably Cl, or C₁-C₁₂ alkyl, R₃: a single bond, C₁-C₁₂ alkylene, C₅-C₆ cycloalkylene or phenylene, R₄: H, alkali metal, ammonium, alkaline earth metal, C₁-C₁₂ alkyl,

and B) hydrolysable silanes containing epoxy groups.
 2. UV-stabiliser mixture according to claim 1, characterised in that the molar ratio of epoxy groups of the silane to the hydroxybenzotriazole of the general formula (1) is greater than 1.4.
 3. UV-stabiliser mixture according to claim 1 or 2, characterised in that one or more of the following hydroxybenzotriazoles are used as the hydroxybenzotriazole of the general formula (1):


4. Process for the production of a UV-stabiliser mixture according to anyone of claims 1-3, characterised in that the hydroxybenzotriazole of the general formula (1) and the hydrolysable silanes containing epoxy groups are mixed and heated to at least 90° C. for at least 30 minutes.
 5. Siloxane system containing a UV-stabiliser mixture according to anyone of claims 1 to
 3. 6. Siloxane system containing a UV-stabiliser mixture obtained according to claim
 4. 7. Siloxane system according to claim 5 or 6, characterised in that the proportion of hydroxybenzotriazole relative to the solids content of the siloxane system is 0.3 to
 20. 8. Siloxane coating material containing a siloxane system according to anyone of claims 5 to 7 and optionally particulate material consisting of a compound selected from the group consisting of oxides, oxide hydrates, nitrides and carbides of Si, Al, Sb and B and of transition metals, preferably Ti, Ce, Fe and Zr, wherein the material has a particle size in the range from 1 to 100 nm.
 9. Use of the siloxane coating material according to claim 8 for coating substrate materials of any kind, preferably thermoplastics, particularly preferably polycarbonates.
 10. Coated material, preferably polycarbonate, characterised in that at least the surfaces of the material exposed to radiation are coated with a siloxane coating material according to claim
 8. 